Pigment dispersion quality in paint has traditionally been evaluated by trained human operators who make visual observations of the surface appearance of a paint “drawdown” sample. The drawdown evaluation typically uses a device known as a “Hegman Fineness Gage”, usually referred to as a “Hegman Gage”, as described in American Society for Testing and Materials (ASTM) Standard D1210 “Standard Test Method for Fineness of Dispersion of Pigment-Vehicle Systems by Hegman-Type Gage”. The Hegman Gage comprises a hardened steel (or stainless steel or chrome-plated steel) block (called a Hegman Gage Block) and a hardened scraper of similar material. The hardened steel block has a flat ground planar surface and has a tapered path machined along its 127 millimeter length. The tapered path is 100 micrometers deep at one end and the path tapers to a depth of zero at its other end. The Hegman Gage for manual drawdowns has a one-half inch wide path. Calibration scales are marked along the lateral edges of the path. Along one edge the scale is marked in micrometers (designating the depth of the tapered path) and along the other edge the scale is marked in “Hegman Units”, ranging from zero to eight, which correspond to a depth of 100 micrometers for a Hegman Unit of zero ranging to a depth of zero for a Hegman Unit of 8.
A paint or paste sample is mixed with a vehicle (usually an alkyd resin) and is prepared using well known methods. A predetermined quantity of paint or paste is deposited at the deep end of the tapered path of the Hegman gage block. The hardened steel scraper is placed on the steel block and drawn along its length, leaving behind, in the tapered path, a film-like deposit of paint whose thickness tapers from a maximum thickness to a minimum thickness.
The operator visually observes the sample and looks for pigment agglomerates that protrude from the paint film surface. These protrusions are known as “particles”, “specks” or “scats”. The operator visually determines the location along the gage where the specks first appear. Because the appearance of the drawdown sample changes as the paste or paint sample begins to dry, a visual observation must be made immediately. Within about ten seconds of the drawdown the operator makes a visual observation of the appearance of the drawdown sample. The operator determines the point along the gage where a definite pattern of specks appear. This point is called the “fineness line” or “fineness measurement” and provides an indication of the fineness or quality of the dispersion of the pigment. The operator also determines the locations of specks and counts them in predetermined ranges along the gage block. Although conducted under controlled lighting conditions these observations are still somewhat subjective.
Because this ASTM D1210 measurement protocol relies upon the expertise of human operators, both for the manual drawing of the hardened steel scraper along the length of the Hegman gage block and for making a visual observation, test results vary from operator to operator and thus lack repeatability. The ASTM measurement results are believed to be more qualitative than quantitative in nature.
Previous devices attempt to provide a quantitative evaluation of pigment dispersion quality in paint. Examples of such devices, which are described in U.S. Pat. Nos. 6,721,055 and 5,249,029, still suffer from certain deficiencies. Both of these devices operate in a so called “dark field” optical mode, i.e., they only collect light scattered from surface anomalies and ignore light reflected from the surface. In a dark field operation, light reflected from the surface of a paint film sample is blocked from reaching a photodetector and only light scattered from pigment agglomerates that protrude above the film surface reach the photodetector. Both of these devices illuminate only a small portion of the paint film sample at a time and require movement of the Hegman gage block (also referred to as a “grindometer block”) past an illuminator and photodetector arrangement, an inherently slow operation that typically takes several seconds. A wet paint film sample begins to dry immediately and changes in appearance as it dries. In the previous devices different portions of a freshly prepared wet film sample are imaged at different times, thus providing different levels of sensitivity on different portions of the paint film samples.
Because of the lack of repeatability of the visual method and the deficiencies of the previous dark field imaging methods, it is believed that there is a need for an improved imaging method and for a more quantitative evaluation of the drawdown sample appearance to provide accurate characterization of pigment dispersion.